Method and apparatus for positioning an object with respect to the isocenter of an acquisition system

ABSTRACT

Certain embodiments of the present invention relate to a system and method for object and isocenter alignment in an imaging system. The system includes an electromagnetic, an electromagnetic receiver, and an imaging unit for determining an isocenter of an imaging scanner based on information from the electromagnetic receiver. The imaging unit identifies the isocenter based on a plurality of electromagnetic position measurements. The imaging unit identifies a center of an object to be imaged based on information from a second electromagnetic receiver. The imaging unit repositions the object based on the isocenter. In an embodiment, the emitter is located on the object, the first receiver is located on a detector, and the second receiver is located on a tool for identifying a center of the object or a portion of the object.

RELATED APPLICATIONS

The present application relates to, and claims priority as a divisionalfrom, U.S. patent application Ser. No. 10/891,664, filed on Jul. 15,2004, entitled “Method and Apparatus for Positioning an Object withRespect to the Isocenter of an Acquisition System,” which in turn claimspriority to French Patent Application No. 0350926, filed on Nov. 28,2003, and entitled “Semi-Automatic positioning of an organ for 3Dreconstruction using EM navigation devices”, each of which isincorporated herein by reference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

MICROFICHE/COPYRIGHT REFERENCE

[Not Applicable]

BACKGROUND OF THE INVENTION

The present invention generally relates to object positioning for imageacquisition. In particular, the present invention relates to objectpositioning with respect to the isocenter of an acquisition system forimage acquisition.

Medical diagnostic imaging systems encompass a variety of imagingmodalities, such as x-ray systems, computerized tomography (CT) systems,ultrasound systems, electron beam tomography (EBT) systems, magneticresonance (MR) systems, and the like. Medical diagnostic imaging systemsgenerate images of an object, such as a patient, for example, throughexposure to an energy source, such as x-rays passing through a patient,for example. The generated images may be used for many purposes. Forinstance, internal defects in an object may be detected. Additionally,changes in internal structure or alignment may be determined. Fluid flowwithin an object may also be represented. Furthermore, the image mayshow the presence or absence of objects in an object. The informationgained from medical diagnostic imaging has applications in many fields,including medicine and manufacturing.

Obtaining an imaging an object or patient using an imaging systemtypically involves exposing the object or patient to a certain amount ofradiation, such as x-ray radiation. The more exposures or image scansexecuted, the greater the radiation exposure of the object or patient.Increased radiation exposure raises health concerns for a patient beingimaged. Additionally, health and safety standards limit radiation dosagefor a patient imaging scan. Health and safety standards may impact imagequality due to reduced or lower quality image scans. Thus, a system thatminimizes radiation dosage and exposure to a patient would be highlydesirable.

Tomographic reconstruction reconstructs tomographic images fortwo-dimensional and three-dimensional image scans. Tomographicreconstruction reconstructs an image from image data projections (suchas x-ray projections) generated in an image acquisition system. Datafrom multiple projections are combined to produce an image describingthe object. Often, two-dimensional slices are reconstructed from scansof a three-dimensional object. The two-dimensional slices may becombined to construct a three-dimensional image.

During tomographic reconstruction, an object or patient organ beingimaged is placed at or near a center of rotation (i.e., the isocenter)of an acquisition system being used (an x-ray source and detector, forexample). For reconstruction of two-dimensional x-ray views acquired ona C-arm imaging system, for example, centering of an object or organ isusually performed under continuous two-dimensional x-ray fluoroscopy.That is, several lateral and frontal projections are taken until anoptimal position has been found from the specific x-ray acquisitions.Therefore, a method of positioning an object or patient organ at or nearthe isocenter of an image acquisition system would be highly desirable.

Therefore, a need exists for an improved object positioning system forpositioning an object at the isocenter of an acquisition system forimage acquisition.

BRIEF SUMMARY OF THE INVENTION

Certain embodiments of the present invention provide a method and systemfor isocenter determination and alignment in a tomographic imagereconstruction system. In a certain embodiment, the method includesacquiring two x-ray projections in an imaging system, determining anisocenter for the imaging system, locating a center position of anobject to be imaged, and positioning the center position of the objectwith respect to the isocenter. The isocenter may be determined based ona segment intersecting the x-ray projections. Electromagnetic navigationdevices, such as electromagnetic emitters and receivers, may be used todetermine the isocenter and locate the center position. The object maybe manually and/or automatically moved to position the center positionwith respect to the isocenter.

In an embodiment, the method includes acquiring at least three positionmeasurements for an electromagnetic receiver attached to a detector inan imaging system and computing an isocenter with respect to anelectromagnetic emitter based on the position measurements. The methodmay also include indicating a center position of an object and movingthe object such that the isocenter and the center position are aligned.The object may be manually and/or automatically moved such that theisocenter and the center position match. The isocenter may be computedbased on a center of the position measurements.

In an embodiment, the method includes calculating an approximate focaldistance in an imaging system, determining position and orientationinformation for an electromagnetic receiver with respect to anelectromagnetic emitter in the imaging system, and identifying anisocenter for the imaging system using the position and orientationinformation and approximate focal distance. The method may also includeback-projecting the position information to identify the isocenter.Additionally, the method may include indicating a center position of anobject around which a tomographic acquisition may be performed andmoving the object such that the isocenter and the center position arealigned. The center position may be indicated using a secondelectromagnetic receiver. The object may be automatically and/ormanually moved such that the isocenter and the center position arealigned.

In a certain embodiment, the system includes an electromagnetic emitterfor generating a magnetic field, an electromagnetic receiver fordetecting the magnetic field from the electromagnetic emitter, and animaging unit for determining an isocenter of an imaging scanner based oninformation from the electromagnetic receiver. The electromagneticreceiver may be located on a detector for acquiring image data. Theelectromagnetic emitter may be located on an object to be imaged.Alternatively, the receiver may be located on the object, and theemitter may be located on the detector. The imaging unit may identifythe isocenter based on a plurality of position measurements from theelectromagnetic receiver. The imaging unit may also identify theisocenter based on a plurality of x-ray projections and a plurality ofposition measurements from the electromagnetic receiver. The imagingunit may reposition an object to be imaged based on the isocenter.

The system may also include a second electromagnetic receiver fordetecting the magnetic field from the electromagnetic emitter. Thesecond electromagnetic receiver may be located on a tool for identifyinga center of an object to be imaged. The imaging unit may identify acenter of an object based on information from the second electromagneticreceiver.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a magnetic resonance imaging system used inaccordance with an embodiment of the present invention.

FIG. 2 illustrates a flow diagram for a method for isocenter positioningof an object in an image acquisition system used in accordance with anembodiment of the present invention.

FIG. 3 illustrates frontal and lateral x-ray projections through anobject to be imaged in accordance with an embodiment of the presentinvention.

FIG. 4 depicts a tool with an electromagnetic receiver pointing to thecenter of an object in accordance with an embodiment of the presentinvention.

FIG. 5 illustrates aligning the isocenter and object center position inaccordance with an embodiment of the present invention.

FIG. 6 illustrates a flow diagram for a method for isocenteridentification and object positioning used in accordance with anembodiment of the present invention.

FIG. 7 illustrates isocenter position with a plurality ofelectromagnetic receivers in accordance with an embodiment of thepresent invention.

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings, certainembodiments. It should be understood, however, that the presentinvention is not limited to the arrangements and instrumentality shownin the attached drawings.

DETAILED DESCRIPTION OF THE INVENTION

For illustration purposes only, certain embodiments of the presentinvention are described in relation to an x-ray imaging system.Embodiments of the present invention may apply to a plurality ofmodalities, such as magnetic resonance (MR) imaging, x-ray imaging,computed tomographic (CT) imaging, electron beam tomographic (EBT)imaging, positron emission tomographic (PET) imaging, single photonemission computed tomographic (SPECT) imaging, and ultrasound imaging.

FIG. 1 illustrates an object positioning system 100 for x-ray imagingused in accordance with an embodiment of the present invention. Thesystem 100 includes an electromagnetic (EM) emitter 110, EM receivers120, 125, an x-ray detector 130, an x-ray source 135, an object 140, atool 150, and a tomographic reconstruction unit 160. The EM emitter 110is attached to the object 140. The EM receiver 120 is attached to thex-ray detector 130. The EM receiver 125 is attached to the tool 150.

The EM emitter 110 and EM receivers 120, 125 are electromagneticnavigation devices. For example, the EM emitter 110 and EM receivers120, 125 may include wire coil trios used to locate a subject based onelectromagnetic fields generated. The EM navigation devices use avariety of methods to locate a subject based on information, such asfield strength and phase. In an embodiment, EM navigation devices may beconfigured according to an industry standard coil architecture (ISCA).

The EM emitter 110 is located on or in, for example, the object 140,such as a patient, organ, or other object to be reconstructed. The EMemitter 110 broadcasts a magnetic field. Characteristics of the magneticfield produced by the emitter 110 may be used to identify the positionof the emitter 110 and, thus, the object 140, with respect to thereceivers 120 and/or 125 in a coordinate system.

The EM receiver 120 is located on or in, for example, the x-ray detector130. The x-ray detector 130 detects rays generated from the x-ray source135. The EM receiver 120 detects a magnetic field from the EM emitter110 on the object 140. The EM receiver 120 transmits data regarding thefield from the EM emitter 110 to the tomographic reconstruction unit160.

The EM receiver 125 is located on or in, for example, the tool 150. Thetool 150 is a pin, clamp, rod, or other implement, for example, that maybe used to point to a center of the object 140 being reconstructed. TheEM receiver 125 detects a magnetic field from the EM emitter 110. The EMreceiver 125 transmits data regarding the field from the emitter 110 tothe reconstruction unit 160.

In an alternative embodiment, the EM receiver 120, 125 is located on theobject 140. The EM emitter 110 is located on the tool 150 or thedetector 130.

The tomographic reconstruction unit 160 (or other imaging unit) receivesdata from the EM receivers 120, 125 to determine the location of theobject 140 and the tool 150 and to determine the isocenter of the system100. The tomographic reconstruction unit 160 aligns the center of theobject 140 with the isocenter of the system 100 to optimize operations,such as three-dimensional image reconstruction. The tomographicreconstruction unit 160 may be implemented in hardware and/or insoftware. The reconstruction unit 160 may be a dedicated processor orpart of a general purpose computer. The reconstruction unit 160 may beembodied in a separate unit or may be combined with other components ofan imaging system.

In operation, a working reconstruction position may be determined usingtwo EM receiver 120 positions with or without x-ray acquisition (frontaland lateral, for example). The source 135 generates x-ray projections atfrontal and lateral positions, for example, through the object 140. Thereceiver 120 and detector 130 identify the frontal and lateralprojections. In an embodiment, information regarding the projections maybe obtained during current and/or prior calibration. The frontal andlateral projection vectors generally do not intersect each other. Anisocenter for the system 100 may be determined at the midpoint of asegment orthogonally connecting the projections. Since the EM receiver120 is attached to the detector 130 and the EM emitter 110 is attachedto the object 140, the isocenter is determined with respect to theobject 140 (the emitter 110).

In an alternative embodiment, a focal distance for the x-ray projectionsmay be approximately determined. Using the approximate focal distanceand EM positioning information, projection vectors and focal points maybe determined without acquiring x-ray images. Once the vectors and focalpoints are determined, the isocenter may be identified from the midpointof the connecting segment, as described above.

After the isocenter has been determined in relation to the emitter 110position, the tool 150 with EM receiver 125 is used to identify a centerposition of the object 140 or portion of the object 140 around whichtomographic acquisition is to be performed. For example, a physicianplaces a clamp with an EM receiver at the center of an organ to bereconstructed in the system 100. The EM emitter 110 and the EM receiver125 may be used to determine the position of the tool 150.

Then, a repositioning system (not shown) is used to reposition theobject 140 such that the center of the object 140 identified above isaligned with respect to the isocenter of the system 100. Therepositioning system may be a manual system and/or an automated system.For example, the repositioning system may be a manual table or supportmoved by a technician until the object 140 center and isocenter areapproximately aligned. Alternatively, for example, a motorized table orsupport may automatically reposition the object 140 based on isocenterand center data.

In an alternative embodiment, the isocenter may be determined withoutradiation (e.g., x-ray) exposures. The detector 130 is positioned asduring a tomographic image acquisition, but x-rays are not produced fromthe source 135. The EM receiver 120 at the center of the detector 130determines three or more positions of the center of the detector 130 inrelation to the emitter 110. The tomographic reconstruction unit 160receives the position data. If three positions are acquired, the threepositions form a triangle. The isocenter may be determined at the centerof the triangle. If more than three positions are acquired, thepositions form a circle. The isocenter may be identified at the centerof the mean circle projected in the mean plane of the acquired detector130 positions.

Alternatively, the reconstruction unit 160 may determine an EMorientation of the acquired detector 130 positions using the emitter 110and receiver 120. An approximate source-to-image distance may bedetermined through calibration or other information. The detector 130positions may then be back-projected. The isocenter is the midpoint of asegment connecting the projections, as described above.

As described above, after the isocenter has been determined, the tool150 identifies the desired center of tomographic reconstruction for theobject 140. The object 140 is manually and/or automatically repositionedsuch that the center is positioned with respect to the isocenter of theimaging system.

FIG. 2 illustrates a flow diagram for a method 200 for isocenterpositioning of an object in an image acquisition system used inaccordance with an embodiment of the present invention. First, at step210, projections or EM receiver 120 positions are acquired (lateral andfrontal, for example). For example, frontal and lateral x-rayprojections from an x-ray source to the x-ray detector 130 are obtainedwith the EM receiver 120 at the x-ray detector 130. Then, at step 220,an isocenter is computed with respect to a position of the EM emitter110. For example, the isocenter is computed based on vectors and focalspots from the x-ray projections. In an embodiment, the isocenter islocated approximately halfway along a segment orthogonally connectingthe x-ray projections.

FIG. 3 illustrates frontal and lateral x-ray projections through anobject to be imaged in accordance with an embodiment of the presentinvention. A lateral focal point F_(l) and a lateral projection vectorv_(l) produce a lateral image at the x-ray detector 130 with EM receiver120. A frontal focal point F_(f) and a lateral projection vector v_(f)produce a frontal image at the x-ray detector 130. The focal pointsF_(l) and F_(f) and vectors v_(l) and V_(f) may be determined throughcalibration. Then, a vector n may be determined from v_(l) and v_(f) asfollows:

$\begin{matrix}{\overset{\rightharpoonup}{n} = {\frac{{\overset{\rightharpoonup}{v}}_{f} ⩓ {\overset{\rightharpoonup}{v}}_{l}}{{{\overset{\rightharpoonup}{v}}_{f} ⩓ {\overset{\rightharpoonup}{v}}_{l}}}.}} & (1)\end{matrix}$Then, locations H_(l) and H_(f) along vectors v_(l) and v_(f),respectively, may be determined from the following equations, forexample:

$\begin{matrix}{{H_{f} = {F_{f} + {\left( \frac{{\left( {{\overset{\rightharpoonup}{v}}_{l} ⩓ \overset{\rightharpoonup}{n}} \right) \cdot {\overset{\rightharpoonup}{F}}_{f}}{\overset{\rightharpoonup}{F}}_{l}}{\left( {{\overset{\rightharpoonup}{v}}_{l} ⩓ \overset{\rightharpoonup}{n}} \right) \cdot {\overset{\rightharpoonup}{v}}_{f}} \right){\overset{\rightharpoonup}{v}}_{f}}}},\mspace{11mu}{and}} & (2) \\{H_{l} = {F_{l} + {\left( \frac{{\left( {{\overset{\rightharpoonup}{v}}_{f} ⩓ \overset{\rightharpoonup}{n}} \right) \cdot {\overset{\rightharpoonup}{F}}_{f}}{\overset{\rightharpoonup}{F}}_{l}}{\left( {{\overset{\rightharpoonup}{v}}_{f} ⩓ \overset{\rightharpoonup}{n}} \right) \cdot {\overset{\rightharpoonup}{v}}_{l}} \right){{\overset{\rightharpoonup}{v}}_{l}.}}}} & (3)\end{matrix}$Using H_(l) and H_(f), the isocenter (I) may be calculated as

$\begin{matrix}{I = {\frac{1}{2}{\overset{\rightharpoonup}{H}}_{f}{{\overset{\rightharpoonup}{H}}_{l\;}.}}} & (4)\end{matrix}$The isocenter is determined with respect to the EM emitter 110 on theobject 140. Alternatively, if an approximate focal distance isdetermined, vectors v_(f) and v_(l) and the focal points F_(f) and F_(l)may be determined using position and orientation information from the EMemitter 110 without acquiring x-ray images.

Next, at step 230, a center position of the object 140 for tomographicacquisition is identified using the tool 150 with the attached EMreceiver 125. FIG. 4 depicts a tool 150 with EM receiver 125 pointing tothe center of an object 140 in accordance with an embodiment of thepresent invention. Then, at step 240, the object 140 is positioned suchthat the center position identified in step 230 is aligned with respectto the isocenter. Positioning may be accomplished manually (with amanual positioning system or table, for example) and/or automatically(with a motorized table or positioning system, for example). Forexample, a fixed motorized table in a vascular C-arm x-ray system maymove a patient until the center of an organ of interest approximatelymatches the isocenter of the C-arm system. FIG. 5 illustrates aligningthe isocenter and object center position in accordance with anembodiment of the present invention. At step 250, tomographicreconstruction of the object 140 may proceed.

In another embodiment, the isocenter of an acquisition system may bedetermined without preliminary scans or projections. FIG. 6 illustratesa flow diagram for a method 600 for isocenter identification and objectpositioning used in accordance with an embodiment of the presentinvention. First, at step 610, three or more positions of the EMreceiver 120 attached to the x-ray detector 130 are obtained. The EMreceiver 120 allows the center of the detector 130 to be recorded duringa tomographic acquisition without x-ray projections, for example. Thethree positions of the detector 130 form vertices of a triangle.

Then, at step 620, the isocenter is located based on the triangle. In anembodiment, the isocenter corresponds to the center of the triangle. Theisocenter (I) may be determined in the plane of the triangle by solvingthe following system of equations:

$\begin{matrix}{{{\begin{matrix}{x^{2} + y^{2}} & x & y & 1 \\{x_{1}^{2} + y_{1}^{2}} & x_{1} & y_{1} & 1 \\{x_{2}^{2} + y_{2}^{2}} & x_{2} & y_{2} & 1 \\{x_{3}^{2} + y_{3}^{2}} & x_{3} & y_{3} & 1\end{matrix}} = 0},} & (5)\end{matrix}$where

$\quad{\begin{pmatrix}x_{1} \\y_{1}\end{pmatrix},\begin{pmatrix}x_{2} \\y_{2}\end{pmatrix},\mspace{11mu}{{and}\mspace{11mu}\begin{pmatrix}x_{3} \\y_{3}\end{pmatrix}}}$are coordinates of the three detector 130 positions expressed in theplane of the triangle.

If more than three detector 130 positions are acquired, the isocentercorresponds to the center of a mean circle or ellipse projected in amean plane of the acquired detector 130 positions. First, the mean planecorresponding to the acquired positions is determined by minimizing thefollowing criteria:

$\begin{matrix}{{\sum\limits_{i = 0}^{n}\;{{{ax}_{i} + {by}_{i} + {cz}_{i} + d}}},} & (6)\end{matrix}$where

$\quad\begin{pmatrix}x_{i} \\y_{i} \\z_{i}\end{pmatrix}$represent the acquired three-dimensional positions of the center of thedetector 130. Then, the mean circle is computed using projectedpositions

$\quad\begin{pmatrix}x_{i} \\y_{i}\end{pmatrix}$of the acquired positions in the mean plane. The mean circle isdetermined by minimizing the following criteria:

$\begin{matrix}{\sum\limits_{i = 0}^{n}\;{{{\left( {x_{i} - c_{x}} \right)^{2} + \left( {y_{i} - c_{y}} \right)^{2} - R^{2}}}.}} & (7)\end{matrix}$The mean ellipse is determined using the following equation:

$\begin{matrix}{\sum\limits_{i = 0}^{n}\;{{{\frac{\left( {x_{i} - c_{x}} \right)^{2}}{a^{2}} + \frac{\left( {y_{i} - c_{y}} \right)^{2}}{b^{2}} - 1}}.}} & (8)\end{matrix}$

Alternatively, the isocenter may be determined based on anelectromagnetic orientation of the acquired detector 130 positions andan approximate value of a source-to-image distance. The source-to-imagedistance may be determined through calibration, for example. Then, thepositions may be back-projected to determine the isocenter.

Next, at step 630, a center position of the object 140 for tomographicacquisition is identified using the tool 150 with the attached EMreceiver 125. Then, at step 640, the object 140 is positioned such thatthe center position identified in step 630 is aligned with respect tothe isocenter. Positioning may be accomplished manually (with a manualpositioning system or table, for example) and/or automatically (with amotorized table or positioning system, for example). For example, afixed motorized table in a vascular C-arm x-ray system may move apatient until the center of an organ of interest is aligned with theisocenter of the C-arm system. At step 650, tomographic reconstructionof the object 140 may proceed.

In an embodiment, a “good” working position may be definedmathematically by expressing some properties of the position, forexample. Position properties may differ depending upon the organ orobject being reconstructed. Properties may also vary depending upon thetool used to locate the object center prior to alignment. Certainembodiments provide a variety of methods to determine a good positionfor tomographic reconstruction using EM navigation devices, such as anEM emitter and an EM receiver. A good working position for athree-dimension tomographic acquisition may be easily and quicklyidentified using receiver and transmitter devices connected to adetector and an object, such as a patient. Efficient positiondetermination saves radiation dosage during centering of the object.

Thus, certain embodiments of the present invention position an objectwith respect to the isocenter of an acquisition system to facilitatetomographic image reconstruction. Certain embodiments provide forsemi-automatic positioning of an organ or other object forthree-dimensional reconstruction using EM navigation devices. Certainembodiments provide for positioning for optimal tomographic acquisitionwithout image exposures or with a minimum number of exposures. EMnavigation devices may be used to determine an optimal position forimage reconstruction.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A method for positioning an object at an isocenter of an imagingsystem, said method comprising: acquiring two x-ray projections in animaging system; emitting a magnetic field using an electromagneticemitter; detecting the magnetic field using an electromagnetic receiver;determining an isocenter for the imaging system using the two x-rayprojections and information from the electromagnetic receiver; locatinga center position of an object to be imaged using information from theelectromagnetic receiver, wherein at least one of the electromagneticemitter and the electromagnetic receiver is located on the object to beimaged; and positioning the center position of the object with respectto the isocenter.
 2. The method of claim 1, wherein said determiningstep further comprises determining the isocenter using a segmentintersecting the x-ray projections.
 3. The method of claim 1, whereinsaid aligning step further comprises at least one of manually andautomatically moving the object to position the center position withrespect to the isocenter.
 4. The method of claim 1, wherein theelectromagnetic emitter is located on the object to be imaged and theelectromagnetic receiver is located on an x-ray detector.
 5. The methodof claim 1, wherein the electromagnetic emitter is located on the objectto be imaged and the electromagnetic receiver is located on a toolconfigured to point to the center of the object to be imaged.
 6. Themethod of claim 1, wherein the electromagnetic receiver is located onthe object to be imaged and the electromagnetic emitter is located on anx-ray detector.
 7. The method of claim 1, wherein the electromagneticreceiver is located on the object to be imaged and the electromagneticemitter is located on a tool configured to point to the center of theobject to be imaged.
 8. A method for centering an object in an imagingsystem, said method comprising: acquiring at least three positionmeasurements for an electromagnetic receiver in an imaging system,wherein the electromagnetic receiver is configured to detect a magneticfield and the electromagnetic receiver is located on a detector of theimaging system; and computing an isocenter with respect to anelectromagnetic emitter using the at least three position measurements,wherein the electromagnetic emitter is configured to emit a magneticfield.
 9. The method of claim 8, further comprising: indicating a centerposition of the object; and moving the object such that the isocenterand the center position are aligned.
 10. The method of claim 9, whereinsaid moving step further comprises at least one of manually andautomatically moving the object such that the isocenter and the centerposition match.
 11. The method of claim 8, wherein said computing stepfurther comprises computing the isocenter based on a center of the atleast three position measurements.
 12. The method of claim 8, whereinthe electromagnetic emitter is located on the object to be imaged.
 13. Amethod for determining a working position for tomographicreconstruction, said method comprising: calculating an approximate focaldistance in an imaging system; determining position and orientationinformation for an electromagnetic receiver with respect to anelectromagnetic emitter in the imaging system, wherein theelectromagnetic emitter is configured to emit a magnetic field and theelectromagnetic receiver is configured to detect the magnetic field, andwherein at least one of the electromagnetic emitter and theelectromagnetic receiver is located on an object to be imaged; andidentifying an isocenter for the imaging system using the position andorientation information and approximate focal distance.
 14. The methodof claim 13, wherein said identifying step further comprisesback-projecting the position information to identify the isocenter. 15.The method of claim 13, further comprising: indicating a center positionof an object around which a tomographic acquisition may be performed;and moving the object such that the isocenter and the center positionare aligned.
 16. The method of claim 15, wherein said moving stepfurther comprises at least one of automatically and manually moving theobject such that the isocenter and the center position are aligned. 17.The method of claim 13, wherein the electromagnetic emitter is locatedon the object to be imaged and the electromagnetic receiver is locatedon a detector of the imaging system.
 18. The method of claim 13, whereinthe electromagnetic emitter is located on the object to be imaged andthe electromagnetic receiver is located on a tool configured to point tothe center of the object to be imaged.
 19. The method of claim 13,wherein the electromagnetic receiver is located on the object to beimaged and the electromagnetic emitter is located on a detector of theimaging system.
 20. The method of claim 13, wherein the electromagneticreceiver is located on the object to be imaged and the electromagneticemitter is located on a tool configured to point to the center of theobject to be imaged.